1. Field of the Invention
The present invention relates generally to an improved method and apparatus for erasing dirty spots in the information content of image reproducing systems including copying systems and printing systems.
2. Discussion of the Background
The present invention is related to the area of analyzing the images of documents and determining compensation filter selections to optimize the reproduction fidelity of the print. FIG. 13 illustrates a general image processing unit (IPU) 10 which has been developed for use in an image forming apparatus such as, scanners, copying machines, facsimile machines, and so forth. The IPU 10 of FIG. 13 typically adopts a mesh-screen image seek operation and an isolated-pixel seek operation in order to erase dirty spots in the image data. The mesh-screen image seek operation is to distinguish ordinary images from mesh-screen images. The mesh-screen images are typically used with images of newspapers, for example.
In a practical application, the IPU 10 is provided with two selectable image reading modes relative to the mesh-screen image seek operation. One is a character image mode and the other is a character/photo mixed-image mode. The character image mode will be used typically when the document to be read includes mainly characters and the character/photomixed-image mode will be used typically when the document to be read includes characters and images such as photos.
A mesh-screen image, such as an image of newspaper, typically appears with images to be read in the character/photo mixed-image mode but not in the character image mode. Accordingly, the mesh-screen seek operation does not need to be operable in the character image mode in a general image processing apparatus including the IPU 10 of FIG. 13.
The dirty-pixel seek operation is used to distinguish a pixel that is not at a white level and is isolated from ordinary pixels. Such a pixel (referred to as an isolated-pixel) is regarded as an undesired dirty spot on a document. Therefore, it may be desirable if such a dirty-spot is erased from the document.
For these operations, the IPU 10 is provided with a mesh-screen image seek circuit 12 and an isolated-pixel seek circuit 13. In the IPU 10, a shading correction circuit 11 corrects the image data of a document which is transmitted from a CCD (charge coupled device) 9 through a video processing unit (VPU) 10a. The image data is then applied to a plurality of circuits such as the mesh-screen image seek circuit 12, the isolated-pixel seek circuit 13, an MTF (Modulation Transfer Function) correction filter 14 and a smoothing filter 15.
The isolated-pixel seek circuit 13 judges whether each of the individual pixels of an image is surrounded by white pixels, either in the character image mode or the character/photo mixed-image mode. A pixel surrounded by white pixels is referred to as the above-described isolated-pixel. When an isolated-pixel is found, the isolated-pixel seek circuit 13 outputs an isolated-pixel signal to a selector 17.
The mesh-screen image seek circuit 12 operates only in the character/photo mixed-image mode and judges whether the image is composed of a plurality of groups of spots, or a mesh-screen image such as in newspapers. The mesh-screen image seek circuit 12 outputs a mesh-screen image signal to a selector 18, indicating the pixel being transmitted is either a mesh-screen image or an ordinary image.
The isolated-pixel seek circuit 13 uses a 5- by 5-pixel matrix, for example, as illustrated in FIG. 4, and picks up the values of a center pixel D22 and its surrounding pixels in the matrix, as illustrated in FIG. 7, and compares these values with a predetermined threshold value X. When the values of the center pixel D22 and all of the surrounding pixels in the matrix are smaller than the predetermined threshold value X, the isolated-pixel seek circuit 13 judges the center pixel D22 to be an isolated and potentially black pixel, or a potential dirty spot. At this time, the isolated-pixel seek circuit 13 outputs an isolated-pixel signal for indicating that the pixel being transmitted is either an isolated-pixel or an ordinary pixel. When the pixel being transmitted is judged to be an isolated-pixel, this pixel needs to be changed to a white pixel, so that the isolated-pixel may be erased.
The selector 17 selects the data from a white data register 16 when the pixel D22 is judged to be an isolated and potentially black pixel, or a potential dirty spot, on the basis of the isolated-pixel signal output from the isolated-pixel seek circuit 13. The selector 17 selects the correction data from the MTF correction filter 14 when the pixel D22 is judged not to be an isolated and potentially black pixel, or an ordinary image spot, on the basis of the isolated-pixel signal output from the isolated-pixel seek circuit 13.
The selector 18 selects data output from either the selector 17 or the smoothing filter 15 on the basis of the mesh-screen image signal output from the mesh-screen image seek circuit 12. In the character mode, however, the selector 18 selects only the data output from the selector 17 regardless of the mesh-screen image signal. In the character/photo mixed-image mode, the selector 18 selects the correction data output from the smoothing filter 15 when the target pixel is judged as a pixel in a mesh-screen image. Also, in the character/photo mixed-image mode, the selector 18 selects the data output from the selector 17 when the target pixel is judged as a pixel not in a mesh-screen image. The data selected by the selector 18 is applied to a .gamma.-correction circuit 19 and then to the following circuit.
Thus, in the character mode, the image data selected is either the white data from the white data register 16 or the MTF correction data from the MTF correction filter 14, according to the isolated-pixel signal from the isolated-pixel seek circuit 13, and the smoothing filter data from the smoothing filter 15 is not selected as the image data. In the character/photo mixed-image mode, the smoothing filter data from the smoothing filter 15 is determined as the image data when the target pixel is judged as a pixel in a mesh-screen image. On the other hand, when the target pixel is judged as a pixel not in a mesh-screen image in the character/photo mixed-image mode, the white data from the white data register 16 is selected as the image data when the target pixel is judged as an isolated-pixel, and the MTF correction data from the MTF correction filter 14 is determined as the image data when the target pixel is not judged as the isolated-pixel.
That is, the IPU 10 of FIG. 13 executes only the isolated-pixel seek operation but not the mesh-screen image seek operation when in the character mode. This means that in the character mode a target pixel in a mesh-screen image may be undesirably judged as an isolated and potentially black spot, or a potential black spot, through the dirty-spot seek operation. A countermeasure for this issue is to set the predetermined threshold value X to a relatively high value for a target pixel in a mesh-screen image. However, this countermeasure has a further drawback in which a real dirty spot in a mesh-screen image might not be found and erased with such a relatively high value X.
On the contrary, if the threshold value X is set to such a value that the isolated-pixel seek operation functions on the mesh-screen image, a problem may occur in which many pixels of the mesh-screen and low contrast images may be erased. Furthermore, if the size of the matrix of FIG. 4 is made larger in order to avoid the above-mentioned problem, another problem may occur that the size of the IPU 10 may become larger as well.
Japanese Patent Laid-open Publication 7-30751/1995 discloses an image processing apparatus that attempts to prevent upsizing thereof by combining a shift register that outputs matrix data to the isolated-pixel seek circuit 13 and a shift register that outputs different matrix data to an edge separation circuit (not shown).
It is believed that there is no image processing apparatus that is capable of properly performing the dirty-spot erasing operation in the character and character/photo mixed-image modes without causing an enlargement of the unit size.